The present invention relates to a method for manufacturing calcium fluoride (“CaF2”) single crystal and CaF2 single crystal. The present invention also relates to an optical system that uses, as a light source, ultraviolet light from excimer laser, such as KrF, ArF, F2 and Ar2, and an exposure apparatus for manufacturing semiconductors and a device fabrication method.
No end to a demand for higher integration of semiconductor integrated circuits has required higher level performance for an exposure apparatus, particularly that of a projection optical system. While resolution in the exposure apparatus may improve along with an increased numerical aperture (“NA”) of the projection optical system, the higher NA makes a depth of focus smaller. Thus, NA cannot be increased beyond a predetermined amount, and a shorter wavelength is required for higher resolution.
While the excimer laser, such as KrF (having a wavelength of 248 nm), ArF (having a wavelength of 193 nm) and F2 (having a wavelength of 157 nm), has been regarded as a prospective light source for future exposure apparatuses for this reason, most conventionally used glass materials are not compatible with a shortened wavelength of the light source. Fluoride crystal is used as a lens material, and CaF2 crystal etc. have been developed. A single crystal type CaF2 is used for exposure apparatuses so as to eliminate influence of grain boundary and crystal orientation; CaF2 single crystal having a predetermined size generally grows in a single crystal growth furnace.
While CaF2 for exposure apparatuses requires high quality in aspects of light transmission performance, durability, large aperture, uniform refractive index, birefringence, etc., the most concerned issue is to reduce birefringence. Preferably, the birefringence of CaF2 has a difference in optical path is below 1 nm cm for exposure apparatuses.
It is known that annealing after crystal growth is effective in reducing birefringence in grown CaF2 single crystal. This process may reduce the birefringent index by heating and maintaining CaF2 at high temperature above 1000° C. However, when a cooling process after annealing provides rapid cooling, the birefringent index disadvantageously increases again and thus the cooling rate should be made low. On the other hand, the low cooling rate itself would lead to long processing time and remarkably deteriorate productivity. Therefore, an optimization of the cooling rate is important.
Optimal cooling rates for various sized CaF2, which have been calculated by empirical rules to maintain low birefringent index, are disclosed in Japanese Patent Applications Publications Nos. 11-240798 and 11-240787. In this case, such cooling is also disclosed as means for increased productivity which combines some cooling rates that have been empirically obtained for respective temperature regions, by paying attention to a fact that an increase of the birefringent index at low temperature is less than at high temperature even when CaF2 is cooled at a large cooling rate.
However, an approach that uses an empirical rule to obtain cooling rates that do not cause birefringence for various differently sized CaF2 is disadvantageous in that it is difficult to stably provide manufactured CaF2 with good quality. In addition, it is necessary to arduously determine proper cooling rates for each size of CaF2 single crystal to be processed. Problematically, it is difficult to confirm that no birefringence occurs after processing, the quality becomes instable, and excessive low cooling rate would lower the productivity.
In addition, the birefringence would occur and the productivity would lower without proper cooling even in a process that associates with heating other than annealing after CaF2 single crystal growth. The conventional empirical approach has been hard to design a process that serves to stably provide low birefringence and high productivity.